130 WHEELER. [Vol. VIII. 



the case with the cavities of the terminal ampullae which are 

 never obliterated. 



In seeking for some clue to the true nature of the coelomic 

 diverticula one naturally turns to Pcripatus. Unfortunately 

 the two accounts of the development of the nephridia and genital 

 ducts in this curious Arthropod — the one by v. Kennel ('85 

 and '88), the other by Sedgwick ('85 and '88) — contradict each 

 other in many particulars. Both authors, however, agree in 

 deriving the mesodermal portion of the nephridium from hollow 

 diverticula of the somites (similar to those seen in the Orthop- 

 tera, in that they extend into the appendages ! ), and both agree 

 in regarding the sexual ducts as modified nephridia. But Sedg- 

 wick derives the nephridium from the portion of the diverti- 

 culum located in the appendage, while Kennel derives it from 

 the inner lower angle at the base of the somite. According to 

 Kennel only the funnel-portion arises in this way, the long 

 duct being formed by a tubular invagination of the ectoderm. 

 On the other hand, Sedgwick derives the funnel and the greater 

 portion of the duct from the mesoderm and believes that only a 

 very small portion of the duct arises by invagination from the 

 ectoderm. These differences apply, of course, to the sexual 

 ducts as well. According to v. Kennel's account not only their 

 unpaired terminal portion (opening in his form on the ante- 

 penultimate segment) but also the deferent ducts and uteri 

 are ectodermal ; only a short piece, corresponding to the 

 nephridial funnel, and uniting the uteri to the ovaries, and the 

 deferent ducts to the testes, has a mesodermal origin. Accord- 

 ing to Sedgwick the dorsomedian portion of the coelom persists 

 in the segments caudad to the fifteenth and is constricted off 

 from the remainder of the somite. The dissepiments are 

 broken down between the adjacent abstricted portions of the 

 somites, so that a hollow tube is formed on either side. These 

 tubes receive the germ-cells from the entoderm and form the 

 sexual glands.^ In the segment bearing the anal papillae 



1 Sedgwick claims that the germ-cells originate in the entoderm and later on 

 migrate into the coelomic wall. In this particular I prefer to adopt v. Kennel's 

 account, according to which the germ-cells have a mesodermal origin, since it 

 accords better with the facts of Annelid development and with my o^vn 

 observations. 



